completion: disable dwim on "git switch -d"

completion: disable dwim on "git switch -d"

Even though dwim is enabled by default, it will never be done when
--detached is specified. If you force "-d --guess" you will get an error
because --guess then implies -c which cannot be used with -d. So we can
disable dwim in "switch -d". It makes the completion list in this case a
bit shorter.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>